Fixes #5587: `json_encode` is now used with `JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE` where it makes sense, also it is now default for `Json::encode()`

I'm find error

```php

Wrong
 } elseif (!empty($this->mimeTypes) && !in_array(FileHelper::getMimeType($file->tempName), $this->mimeTypes, true)) {

Correct
} elseif (!empty($this->mimeTypes) && !in_array(FileHelper::getMimeType($file), $this->mimeTypes, true)) {
```

Not need call '$file->tempName' because getMimeType get object $_FILE => $file, so use $file and validator works fine

issue #2852

fix tabs

Fixes #1412: `FileValidator` still triggers `uploadRequired` error in some case when `skipOnEmpty` is true and no upload is provided

fixes #991

fixes #845